The present invention deals with the field of turbomachine engineering. It relates to a rotor for a turbomachine in accordance with the preamble of claim 1 and to a process for producing a rotor of this type.
The efficiency of a rotating thermal machine, for example of a gas turbine or the like, can be improved significantly by raising the operating temperature, for example to over 600xc2x0 C. Rotors for use at temperatures of over 600xc2x0 C., for example, should be produced from particularly creep-resistant materials, so that there is no need for complex rotor cooling. This applies in general to rotors for gas turbines, steam turbines, turbochargers, compressors and pumps. As the operating temperature increases, it is necessary to switch from low-alloy steels to higher-alloy martensitic steels, to austenitic steels and to nickel-based alloys, so that sufficient creep rupture strength is always ensured. However, nickel-based alloys are considerably more expensive than low-alloy steels. Therefore, to minimize the costs of a rotor, only the high-temperature region or section of the rotor should consist of the particularly creep-resistant materials, while the remaining regions (e.g. the rotor ends) can be produced from a suitable steel.
The production of a rotor of this type, which is usually welded together from a plurality of segments in disk or drum form, accordingly requires a join between different materials. The join can be produced by screwing or welding the parts together. A screw connection is exposed to excessive mechanical stresses. Although it has already been used for relatively small turbines, there is no experience of such arrangements in large turbines.
As has already been mentioned, when the parts of the rotor are being welded together, not only are welded joints between two elements made from the same material required, but also there is a need for welded joints between different materials. While the welded joints between elements made from the same material are relatively uncritical, problems arise when welding together elements made from different materials: the solidification of the weld pool under certain circumstances leads to the formation of cracks.
U.S. Pat. No. 4,333,670 has proposed the use of a transition joint, which involves arranging a multiplicity of transition parts which have been welded together, each have a specific material composition and allow stepwise matching of the two tubular parts which are to be joined to one another, for the purpose of joining tubular parts made from a low-alloy carbon steel and a high-temperature alloy with a high chromium content. However, the production of a transition joint of this type, comprising numerous parts which have been welded together, is extremely complex, in particular in the case of large dimensions. Furthermore, a transition joint of this type takes up a considerable amount of space between the parts which are to be joined, and this space is not available in a rotor.
In U.S. Pat. No. 4,743,165, a special welding process (xe2x80x9cinertia weldingxe2x80x9d) is used during the production of a rotor of a gas turbine in which disks of a superalloy (IN100) are welded to sealing parts made from a different alloy (IN718). There is no provision for parts made from a superalloy to be joined to parts made from a steel.
Finally, it is proposed, in U.S. Pat. No. 4,486,385, for annular elements which consist of different steels to be joined by welding through the use of a tubular transition piece which is produced by powder metallurgy methods and has a composition which varies over its length and at each end corresponds to the composition of the respective annular element which is to be joined. There is no provision for a welded joint between steel parts and parts made from a nickel-based alloy.
It is an object of the invention to provide a rotor for a turbomachine which, even without complex cooling, is suitable for elevated operating temperatures and is welded together, without loss of mechanical strength, from parts which consist of particularly creep-resistant and less creep-resistant materials, and to describe a process for producing a rotor of this type.
The object is achieved by the combination of features described in claims 1 and 10. The essence of the invention consists in providing, for the welded joint between two parts or sections, of which one consists of a less creep-resistant material and the other consists of a particularly creep-resistant material, a transition region which has been produced by powder metallurgy and, on the side at which the transition region is welded to one of the parts or sections, has precisely the composition of the adjoining part or section. The fact that the transition region, on the respective side of the welded joint, is matched, in terms of its composition, to the part which is welded on results in a secure welded joint which can withstand high mechanical loads. The transition region can be produced in a space-saving and relatively simple manner.
According to a first embodiment of the invention, the transition region is welded to one part on only one side, while on the other side it is joined to the other part of the joint by powder metallurgy.
In particular, the transition region is a layer of a less creep-resistant material, the composition of which corresponds to that of the second rotor section. The layer is joined to the first rotor section by powder metallurgy and is welded to the second rotor section. The first rotor section may optionally likewise be produced by powder metallurgy or may be forged.
However, it is also possible for the transition region to be a layer of a particularly creep-resistant material, the composition of which corresponds to that of the first rotor section. The layer is in this case joined to the second rotor section by powder metallurgy and is welded to the first rotor section.
According to a second preferred embodiment, the transition region is formed by a separate transition piece which is welded to both rotor sections, the composition of the transition piece being position-dependent and, on the side facing the first rotor section, corresponding to the composition of the first rotor section and, on the side facing the second rotor section, corresponding to the composition of the second rotor section.
In particular, the transition piece may be in the form of a ring or an unperforated or perforated disk.
The less creep-resistant material may be steel while the particularly creep-resistant material may be a nickel-based alloy.
A first preferred embodiment of the process according to the invention is characterized in that the transition region is a layer of a less creep-resistant material, which corresponds to the second rotor section, and in that the layer and the first rotor section are produced together by powder metallurgy.
A second preferred embodiment of the process according to the invention is characterized in that the transition region is a layer of a less creep-resistant material, which corresponds to the second rotor section, and in that the layer is applied to the first rotor section by powder metallurgy.
A third preferred embodiment of the process according to the invention is characterized in that the transition region is a layer of a particularly creep-resistant material, which corresponds to the first rotor section, and in that the layer and the second rotor section are produced together by powder metallurgy.
A fourth preferred embodiment of the process according to the invention is characterized in that the transition region is a layer of a particularly creep-resistant material, which corresponds to the first rotor section, and in that the layer is applied to the second rotor section by powder metallurgy.
A further embodiment of the process according to the invention is distinguished by the fact that, to form the transition region, first of all a separate transition piece is produced by powder metallurgy, which transition piece has a position-dependent composition, such that the composition, on the side facing the first rotor section, corresponds to the composition of the first rotor section and, on the side facing the second rotor section, corresponds to the composition of the second rotor section, and that the transition piece is then welded to the two rotor sections.
To test the weld seams, it is advantageous if, first of all, the transition piece is welded to the first rotor section, and the associated weld seam undergoes nondestructive testing, in particular by means of X-rays, before the transition piece is welded to the second rotor section.
Further embodiments will emerge from the dependent claims.